Resistance to chemotherapy and radiation is a big obstacle for patients with glioblastoma multiforme (GBM) Therefore, it is critical to understand mechanisms responsible for chemo-radioresistance these patients. Our recent findings led us to hypothesize that the histone methyltransferase (HMT) MMSET is a contributor to chemoradioreslstance. MMSET (also called WHSC1, NSD2) has been shown to methylate histone H4 Lys20 (H4K20), H3 Lys27 (H3K27) and H3 Lys36 (H3K36). Besides several reports linking it to transcriptional regulation, the cellular function of MMSET remains obscure. We found that MMSET participates in the ATM-MDC1-53BP1 pathway during the DNA damage response. Specifically, MMSET accumulates at sites of DNA damage. Correlating with this, H4K20 methylation, which is required for 53BP1 recruitment [1], also increases at the sites of DNA damage. Downregulation of MMSET decreases H4K20 methylation and abolishes the accumulation of 53BP1 to the sites of DNA damage. These results suggest that MMSET functions as an upstream regulator of 53BP1 through its HMT activity. In support of its role in DNA damage responses, MMSET affects cellular sensitivity to temozolomide (TMZ) and radiation (RT). Finally, we found that MMSET is overexpressed in a subset of glioblastoma lines, and overexpression of MMSET is associated with resistance to temozolomide (TMZ) and radiation (RT). Based on these preliminary findings, we hypothesize that MMSET regulates 53BP1 and the TMZ/RT response, and that misregulation of MMSET could affect GBM sensitivity to chemo-radiotherapy (Figure 1). To further examine this hypothesis, we propose the following Specific Aims: 1. Structure-function analysis of MMSET in the DNA damage response; 2. Investigate the role of MMSET in TMZ/RT response using mouse models; 3. Examine the expression of MMSET in GBM patients and its correlation with TMZ/RT response. These studies will reveal a novel role of MMSET in cellular response to genotoxic stress. Furthermore, the planned translational studies will define whether MMSET is an important modulator of treatment response for glioblastoma patients, which would provide key insight into why at least some patients with a favorable MGMT methylation status fare poorly with chemo-radiotherapy.